The quest for an AIDS vaccine has suffered another setback. The National Institutes of Health announced April 25 that it had stopped immunizing volunteers with the experimental vaccine known as HVTN 505 because it had become clear that the vaccine doesn’t prevent HIV infection.

Since the trial began in 2009, 1,250 volunteers had received the vaccine and 1,244 others had gotten a placebo — both as a series of shots over 24 weeks. Most participants were men who have sex with men. Among volunt...